Direct IO still does read ahead caching, so they both actually use the cache as I understand it. If you have filesystemio_options set to "directio", or "setall" in oracle to bypass the os cache then perhaps cache IO would have a performance edge when writing to disk, though I haven't tested it myself. If you're not running oracle using directIO, or setall, and have filesystemio_options set to "async" then I would advise against cache IO on the controller, as you would already be double caching with the OS, and oracle.
-----Original Message-----
From: linux-poweredge-bounces at dell.com [mailto:linux-poweredge-bounces at dell.com] On Behalf Of mcclnx mcc
Sent: Thursday, May 11, 2006 5:11 PM
To: linux-poweredge at lists.us.dell.com
Subject: direct IO or cache IO?
we have several DELL 6800 with LSI 4e/DC adapter cards
(128 MB cache) in it. DELL 6800 run under Redhar AS
4.0 and ORACLE 9ir2. When I configured DELL 6800
server local disks (Mirror or RAID 5), it have two I/O
policy to choice - direct IO and cache IO.
I doing simple test by copy files between differents
disk set and found "direct IO" performance better
than "cache IO". It is against what I read before. I
used to saw documents from SUN and IBM said "large
disk array cache will improve I/O performance".
Any one has suggestion FOR ORACLE server which I/O
policy should I setup?
Thanks.
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